Impairments in mitochondrial functions have been frequently implicated in ischemic brain injury associated with stroke or cardiac arrest. However, the extent to which mitochondrial dysfunction in neurons and glia contribute to neurodegeneration is unknown and the mechanisms leading to mitochondrial failure are elusive. Mitochondrial impairment can result from activation of the permeability transition pore or excessive mitochondrial fission leading to loss of matrix pyridine nucleotides (NAD+, NADP+) and consequent detrimental NAD+ catabolism. We hypothesize that the major cellular NAD-regulating enzyme CD38 can significantly contributes to intracellular NAD+ hydrolysis following an ischemic insult and that inhibition of this enzyme will dramatically ameliorate the ischemic brain injury. This notion is strongly supported by our preliminary data that suggest promising protection against ischemic brain damage by nicotinamide mononucleotide (NMN), a naturally occurring compound that inhibits CD38 NAD+ glycohydrolase and also feeds into the NAD+ salvage pathway. The primary goal of this study is to determine whether pathologic morphological changes of neuronal or astrocytic mitochondria precedes brain tissue NAD+ depletion and, whether neuronal or astrocytic activity of CD38 is a major contributor to NAD+ hydrolysis following ischemia. To address these questions we propose to: 1. Utilize our unique transgenic animals that express fluorescent marker proteins specific either to neuronal or to astrocytic mitochondria. These animals will be used to quantify mitochondrial morphometric alterations specifically in neurons or astrocytes in brain. 2. To determine the specific role of CD38 in post-insult NAD+ catabolism we will utilize a CD38-null mice. The role of CD38 in cell death of astrocytes and neurons will be examined by exposing the pure neuronal and astrocytic cell culture to oxygen/glucose deprivation and by subjecting CD38 deficient animals to transient forebrain ischemia. 3. Examine the mechanisms of NMN protection against ischemic damage. We will perform both dose-dependent and time-effect studies with NMN administration following ischemic insult. After the designated recovery period, the histological and neurological outcome will be examined. The significance of this work is that it proposes both mechanistic and translational approaches to unravel the mechanisms of neuronal and astrocytic NAD+ catabolism and determine its role in acute brain injury. Furthermore, the identification of NMN protective mechanisms will significantly impact the clinical application of NAD+ precursors as therapeutic compounds for acute brain injury as stroke and TBI or chronic neurodegenerative disease.